Circuitry to monitor an inductive circuit

ABSTRACT

A circuit to monitor a short circuit (or a line interruption) in an inductive circuit which is connected to a signal-processing circuit through a high-ohmic filter circuit. The monitoring circuit is formed as a component part of or as an additional element to the signal-processing circuit and initiates a test cycle to determine the inductance of the circuit to be monitored when, for example, the ignition of an automotive vehicle is turned on when the inductive circuit is used in the circuitry for sensing wheel rotation behavior of an automotive vehicle.

This application is the U.S. national-phase application of PCTInternational application Ser. No. PCT/EP93/01569.

BACKGROUND OF THE INVENTION

The present invention relates to a circuit for monitoring an inductivecircuit which is formed as part of a signal-processing circuit or as anadditional element and which is connected, through a high-ohmic inputcircuit or filter circuit, to the signal-processing circuit. Themonitoring initiates a test cycle to determine the inductance of thecircuit to be monitored, including the filter circuit, if predeterminedconditions apply and/or in predetermined intervals.

A monitoring circuit of this type is disclosed in EP 0 358 887 A1. Inthis monitoring circuit, the duration of a test signal passing throughthe inductive circuit to be monitored is monitored and evaluated withrespect to proper duration by means of a time-difference measuringapparatus. To this end, the time difference of the duration of a signal,which is conducted to the time-difference measuring apparatus throughthe inductance and a time element, is compared to the signal which isconducted directly through an equal time element to the measuringapparatus.

Circuits of this type are particularly suitable to detect short circuitsin the inductive transducer of a wheel sensor. If the short circuit isin the line leading to the transducer, in its input circuit or filtercircuit, the short circuit is likewise detected in the monitoringoperation. A line interruption is also detected. Sensors of this type,which are required in anti-lock systems or traction slip control systemsof automotive vehicles, for example, are safety-critical component partswhich should be checked permanently for operability, short circuits orline interruptions. In a low-ohmic input circuit or filter circuit, ashort circuit may be detected relatively easily by determining the ohmicresistance between an output of the filter circuit and ground. For ahigh-ohmic filter circuit, such an arrangement is not suitable inpractical operations because the ohmic internal resistance of theinductive circuit is low compared to the resistors in the circuit orinput circuit. Therefore, the voltage drop, which may be measured at theoutput of the filter circuit when a current is applied, will be changedby a short circuit only to such a minor extent that reliable evaluationof the measurement results is not possible.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a circuit formonitoring an inductive circuit which has a simple design, does notrequire additional terminals and permits detecting short circuits orline interruptions in a reliable manner, even if the inductive circuitis connected through a high-ohmic filter circuit or input circuit.

This object can be achieved by circuitry having the special featuresthat, at the beginning of the test cycle, one of the two outputs of thefilter circuit is connected to ground, while the second output isconnected to a voltage source for a predetermined period, and theinductance is determined from the potential variation at a second outputof the filter circuit.

It is expedient that the predetermined period corresponds at least tothe transient time of the circuit to be monitored, including the filtercircuit. Upon lapse of the predetermined time period and disconnectionof the voltage source, the potential variation at the second output isevaluated to determine the inductance.

Also, symmetrically arranged filter circuits may be used, each of whichhas one high-ohmic series resistance in the lines leading from theinductive circuit to the signal-processing circuit, one capacitorinterconnecting both outputs of the filter circuit, and one inputcapacitor which connects one of the outputs of the inductive circuit orone of the inputs of the filter circuit to ground.

To simplify the analysis of the signal, a d.c. voltage potential may beset for such a filter circuit by a voltage divider which is connected toa source of d.c. voltage on one side and to ground on the other side.When a line interruption occurs, an elevated potential differenceresults at the two inputs of the signal-processing circuit. However, thecircuitry according to the present invention also allows detecting aline interruption by the consequent change in the inductance of thecircuit to be monitored.

In another embodiment of the present invention, the monitoring circuitdischarges the capacitor, which interconnects both outputs,instantaneously after or simultaneously with the disconnection of thevoltage source by grounding the second output for a short interval. Theanalysis of the potential variation during the test cycle to determinethe inductance of the inductive circuit is highly facilitated by thisarrangement.

Further features, advantages and possible applications of the presentinvention will be understood from the following description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a circuit diagram of a monitoring circuit for an inductivecircuit constructed in accordance with the present invention and oneembodiment of a filter circuit or input circuit between the inductivecircuit and a signal-processing circuit,

FIG. 2 is a circuit diagram of a modified form of the embodiment of FIG.1, and

FIG. 3 shows a number of waveforms useful in understanding the operationof the circuitry of the embodiment of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an equivalent circuit 1' of an inductive transducer 1, asignal-processing circuit 2 and a high-ohmic input circuit or filtercircuit 3 are shown. The signal-processing circuit 2 includes, in dashedlines, a monitoring circuit 4, being part of the circuit 2, whichinitiates test cycles to determine the inductance of the inductivecircuit 1 and evaluates the signals obtained and applied to the inputterminals C and D of the signal-processing circuit 2.

The equivalent circuit 1' of the inductive circuit 1 to be monitored isformed of the series arrangement of an ohmic resistor RS and aninductance LS. A capacitor CS is connected in parallel to this seriesarrangement. The inductive circuit is, for example, an inductivetransducer of a wheel sensor serving to measure the rotations of awheel. Transducers of this type include coils in which an alternatingvoltage is induced on rotation of the wheel, the frequency and amplitudeof which indicates the rotation. The output signal of the sensor,applied to the terminals A, B, is delivered to the signal-processingcircuit 2, where it is processed and evaluated, through the filtercircuit 3 which usually is positioned at the end of an electrical line.

The filter circuit 3 in FIG. 1 is arranged symmetrically. One seriesresistor R1, R2 is inserted in each of the two supply lines, which ishigh-ohmic as compared to the internal resistor RS of the sensor. Acapacitor C1 interconnects the two outputs C, D of the filter circuit 3.Two further capacitors C2, C3, connected to ground, are provided on theside of the filter circuit 3 connected to the sensor. The capacitors C1,C2, C3, along with the ohmic resistors of the filter circuit and theinternal resistor of the inductive circuit, form a low-pass filter.

Further, the filter circuit 3 according to FIG. 1 includes a voltagedivider with the resistors R3, R4. The voltage divider is connected tothe positive pole of the supply voltage source V_(CC), on the one hand,and to ground GND, on the other hand.

The ohmic resistors R1, R2 are at least roughly identical. In theinactive condition, where no voltage is induced in the sensor 1, almostthe same potential is applied to the terminals A, B, C, D, because theinput terminal C, D of the signal-processing circuit 2 is high-ohmic. Aline interruption in the sensor 1, or in the connecting line or in theinput circuit 3 may be detected by means of the signal-processingcircuit 2.

FIG. 2 serves to explain the mode of operation of the circuitryaccording to the present invention, illustrating the circuit in FIG. 1,in a slightly modified representation, and in which, additionally, threetransistor stages 5, 6, 7 or semiconductor switches are illustrated foran understanding of the function of the test cycles. Switches 5, 6, 7are included in the monitoring circuit 4 of FIG. 1. In the illustrationof the embodiment of FIG. 2, the monitoring circuit 4 is quasi split upinto an evaluating and control circuit 4' and the semiconductor switches5, 6, 7. The outputs of the evaluating and control circuit 4' leading tothe control inputs of the switches 5, 6, 7 also are shown in FIG. 2 andand are identified by CB, CC, CD. Further, identical reference numeralshave been assigned to like parts and like terminals in FIGS. 1 and 2.

The mode of operation of an embodiment of the circuitry according to thepresent invention is explained hereinbelow with reference to FIG. 2 inconjunction with the waveform diagrams in FIG. 3.

Monitoring operations are performed whenever the ignition of the vehicleis switched on, for example. No voltage is induced in the inductivetransducer, the equivalent circuit 1' of which is shown, at this pointin time because the wheels are not yet moving. Practically nothingchanges in this condition when the vehicle rolls slowly.

The test cycle is now started by an output signal CD of the monitoringand control circuit 4'. By means of the transistor 5, the signal CDconnects the terminal D of the signal-processing circuit 2 of FIG. 1 orthe filter circuit 3 to ground GND at the point in time t₀. After ashort transient time, a determined d.c. voltage potential is developedat the second terminal of the signal-processing circuit 2, i.e. at theterminal C, the magnitude of which potential is predetermined by thesupply voltage V_(CC) and by the voltage dividers R3, R4. At the pointin time t₁, a signal CB, which drives the transistor 6 and results inclosing of the semiconductor switch, causes the potential at terminal Cto rise to the level of the energy supply of source V_(CC).

The actual measuring operation, which is appropriate to determine theinductance and to detect short circuits, starts at the point in time t₂which marks the termination of the actuating signal CB and, thus, thedisconnection of the current source V_(CC) from terminal C. The energystored in the inductance LS in the presence of the signal CB, i.e.between the points in time t₁ and t₂, after the switch 6 is opened,results in the current IL being continued and, thus, in influencing thepotential variation at the terminal or input C. The current IL flowingthrough the inductive circuit may be calculated from the ohmic resistorsR1, R2, RS, R3 and R4 in the static condition, that means after theswitch 6 (signal CB) has been actuated and the transient processes havefaded.

A preferred feature in the present embodiment of the invention is thatthe input C is grounded for a very short interval dt₃ after theconnection between the input C and the voltage source V_(CC) has beeninterrupted by way of the semiconductor switch 6. This is done byactuating the switch 7 by means of a signal CC. By this provision, thecapacitor C1 is discharged very quickly to full extent. The current ILthrough the inductance LS remains practically constant during this shortinterval.

The charging of C1 dictates the potential variation at terminal C afterthe short interval dt₃. Because a still higher potential is applied topoint B than to point A, C2 discharges through the resistor RS of theinductive circuit as long as the potential is equal at points A and B.This discharging operation is assisted by the continuously flowingcurrent IL, produced by the inductance LS, and by the now commencingcharging current of capacitor C1. The result is that the potential atpoint B very quickly becomes less than the potential at point A, thecharging of C1 being thereby delayed. After current IL has faded, whichis produced by the energy stored in the inductance, an identicalpotential finally results at terminals A and B.

However, the previously described operations occur only when sensor 1 isintact. In the presence of a short circuit or a line interruption, thecapacitor C1 is charged much quicker. This can be seen by observing andevaluating the potential variation at terminal C. The waveform diagramsin FIG. 3 illustrate this condition. The three top waveform diagramsshow the course of the signals at the terminals CD, CB and CC. Theswitches, i.e. transistors 5, 6 and 7, are closed when an actuatingsignal "1" is applied.

The potential variation at terminal C is also shown in FIG. 3. Thepotential V_(Cstat) develops after actuation of the transistor 5 orgrounding of terminal D of the signal-processing circuit 2. V_(Ct4)refers to the potential which develops at the point in time ofmeasurement t₄ when the sensor and the sensor connection are intact inthe absence of an error. The interval T is so selected, in conformitywith the inductance LS of sensor 1, that the transient process is notyet terminated upon expiry of interval T with respect to pulse dt₃, thatmeans at the point of time t₄, and roughly the maximum discrepancy V ofthe potential at terminal C from the static potential V_(Cstat) occurs,with the sensor intact, that means in the absence of short circuits andline interruptions. If there is a defect, charging of the capacitor C1terminates long before at the point of time t₄, and the potential atterminal C has risen to the value V_(Cstat). The dashed line of thepotential variation V_(C) in FIG. 3 illustrates the conditions in thepresence of a short circuit or a line interruption.

It should be noted for the sake of completeness that a time difference(t₀ -t₁) between the initiation of the signals CD and CB is unnecessary,if subsequently, that means prior to the termination of the signal CBand the almost instantaneous application of the short-time signal CC,the transient process and, thus, the occurrence of static conditions isawaited.

Modifications of the described circuitry and the actuation, as comparedto the embodiments shown in FIGS. 2 and 3, are possible. It is in anycase essential that the energy stored in the inductance of the sensor 1has an effect on the potential variation at an output of the filtercircuit 3 or at a terminal of the signal-processing circuit 2 and isevaluated for the monitoring operation.

We claim:
 1. A circuit for monitoring an inductive circuit, said circuitcomprising:a signal processing circuit; a filter circuit for connectingthe inductive circuit to said signal-processing circuit, said filtercircuit having first and second outputs and two parts with each parthaving a series resistor in a line leading from the inductive circuit tothe signal-processing circuit, wherein said series resistor has a valuesufficiently high such that a change in the voltage drop at said firstand second outputs of said filter circuit upon a short circuit isminimized, said filter circuit providing a reliable measurement signalto said signal processing circuit; a voltage source; means forselectively connecting and disconnecting the first output of the filtercircuit to ground and the second output of the filter circuit to saidvoltage source for a predetermined period of time; and means fordetermining the inductance of the inductive circuit from potentialvariations at the first output of the filter circuit.
 2. A circuitaccording to claim 1 wherein:(a) said connecting means connect thesecond output of the filter circuit to said voltage source forpredetermined period of time which is at least the transient time of theinductive circuit being monitored, including the filter circuit, and (b)said determining means evaluate the potential variation at the secondoutput of the filter circuit to determine the inductance of theinductive circuit upon lapse of the predetermined period of time anddisconnection of the second output from said voltage source.
 3. Acircuit according to claim 2 wherein the filter circuit further has:(a)two inputs, (b) a capacitor interconnecting the first and second outputsof the filter circuit, and (c) each part is symmetrically arranged andfurther has an input capacitor connecting one of the inputs of thefilter circuit to ground.
 4. A circuit according to claim 3 wherein thefilter circuit further includes a voltage divider connected between saidvoltage source and ground and which sets a d.c. voltage potential forthe filter circuit.
 5. A circuit according to claim 4 further includingswitching means for connecting the second output of the filter circuitto ground for a short period of time after said voltage source isdisconnected to immediately discharge the capacitor interconnecting theoutputs of the filter circuit.